1. Field of the Invention
The present invention relates to radiation curable coatings for telecommunications elements such as optical fibers and optical fiber ribbons. More particularly, the present invention relates to colored radiation curable coating compositions for producing cured coatings on a telecommunications element, the coating compositions having an identifying color provided by dye molecules which are covalently bonded to the cured coating.
2. Description of Related Art
For many years now, optical fibers have been used as a reliable transmission medium in telecommunications cables. Typically, an optical fiber comprises a core, a cladding and one or more coatings applied over the cladding.
One purpose of the coatings is to protect the surface of the optical fiber from mechanical scratches and abrasions typically caused by subsequent handling and use. Another purpose of the coatings is to protect the glass from exposure to moisture. The coating or coatings may also have some influence over the fiber's optical characteristics because the coatings are physically responsive to external mechanical forces and temperature. The coating compositions applied to the optical fiber are typically liquid, radiation curable compositions. Typically, the coating compositions are cured on the optical fiber by exposing the coating composition to ultraviolet radiation, electron beam radiation or ionizing radiation for a predetermined period of time deemed suitable for effective curing. Telecommunications cables containing optical fibers come in a variety of configurations. In some cables, the optical fibers are held loosely inside a buffer tube. In other cables, the optical fibers are arranged in a planar array to form an optical fiber ribbon. The planar array is typically encapsulated by one or more radiation curable matrix material layers. The radiation curable matrix layers are cured by exposing the matrix material to ultraviolet radiation, electron beam radiation, ionizing radiation or infrared radiation for a predetermined period of time deemed suitable for effective curing.
In a telecommunications cable containing multiple optical fibers, the optical fibers may be distinguished from each other by the use of a color coating layer which has been applied over a coated optical fiber. Colors in the color coating layer are usually obtained by dispersing colored pigment particles in a suitable liquid carrier and applying the liquid carrier over the coating. Unfortunately, the use of pigment particles to provide color in color coatings for optical fibers has presented manufacturing and performance problems. For example, the pigment particles and the liquid carrier tend to gradually separate into two distinct phases.
As a result, pigmented color coatings have a relatively short shelf life. In addition, the phase separation in a pigmented coloring system is further complicated by the concurrent agglomeration of pigment particles. Undesirably, the presence of pigment particle agglomerates in a color coating on a coated optical fiber can induce micro-bending which results in transmission losses. Typically, a relatively high concentration of pigment material is required to achieve an opaque ultraviolet radiation curable color coating. Unfortunately, the required high concentration inhibits the transmission of incident ultraviolet radiation which is necessary to cure the color coating material because the pigments refract, reflect and scatter the incident radiation. The inhibition of the ultraviolet radiation results in a reduction in processing speed of the optical fiber along a manufacturing line and thereby increases production costs. Also, the slow cure speed of pigmented color coatings causes the processing and the cure of these materials to be sensitive to minor changes in the thickness of the color coatings.
The use of dyes to provide color in color coatings has been considered as an alternative to pigment-based color coatings. Dyes have the advantage over pigments of faster curing because the dyes do not scatter the curing radiation, although some dyes may absorb light which could slow curing. Dyes, however, are generally not preferred because they diffuse (bleed) out into common cable filling compounds resulting in a color loss. In an effort to reduce the bleeding, U.S. Pat. No. 5,074,643 teaches the use of a polymeric dye in a color coating. The polymeric dyes are macromolecular chromophore containing molecules that become entrapped in the cross linked coating network. While the entrapment results in a slowing of the bleeding process, the dyes nevertheless still bleed. Over time, even with the entrapped polymeric dyes, the color imparted to the fibers is likely to be lost and if the color is lost from the fibers, then identification of each of the fibers becomes extremely difficult and time consuming in the field during fiber splicing.
The color imparted to the fibers will be lost over time if the dyes themselves lack stability. In particular, the dyes should have sufficient thermal stability and light fastness to impart the desired color for an extended period of time.
If a telecommunications cable has many optical fiber ribbons, it is generally desirable to distinguish one optical fiber ribbon from another by coloring each of the optical fiber ribbons. Typically, color in a colored optical fiber ribbons is obtained in the same way as color is obtained in a color coated optical fiber. Either the optical fiber ribbon matrix composition is provided with pigments or a polymer dye is used. The same problems mentioned above with respect to colored optical fibers apply to colored optical fiber ribbons.
It is desirable to provide a durable color coating for a telecommunication element, such as an optical fiber, that can withstand the conditions in a typical operational environment that such elements are typically found. The present invention provides such a durable coating.